In recent years, performances of electronic devices have improved and miniaturization and portabilization of electronic devices have progressed with the advancement of electronic technologies, and the demand for a secondary battery having a high energy density as a power supply has increased. Examples of a secondary battery include, for example, a nickel hydrogen secondary battery, a lithium ion secondary battery and the like. For these secondary batteries, developments of articles having a high capacity and a high life span are also in progress along with miniaturization and weight reduction of the devices.
An electrode of a secondary battery is composed of an electrode-active material, a conductive auxiliary agent, and further a binder that attaches them to a current collector. As a binder resin for a secondary battery, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene were conventionally used both for a positive electrode and a negative electrode (non-Patent Literatures 1 and 2). However, a positive electrode or negative electrode may repeat volume expansion or contraction at the time of charge and discharge in a secondary battery, which causes an active material or conductive agent to drop off, and thus the cycling life of charge and discharge may be shortened. Therefore, a binder for an electrode is required to have a cushioning property to resist swelling or contraction of the electrode. However, the fluorine resin is insufficient in the cushioning property that allows fitting to the electrode. Furthermore, a fluorine resin also has poor adhesion to a current collector or active material, and shows no effects as a binder unless it is used in a large amount. For this reason, a fluorine resin could not increase density of an active material in an electrode, which has inhibited manufacturing a battery electrode which has a high capacity. Furthermore, a fluorine resin also has problems that it characteristically dissolves in a specific solvent such as N-methylpyrrolidone, and that it has a bad influence on the human body and the environment such as an unusual odor at the time of manufacturing an electrode and the like.
Regarding these problems, in Patent Literature 1, a binder resin is obtained by adding a cross-linking agent to an acrylate resin dissolved in a solvent, and reacting the resin and the cross-linking agent in heating and pressing steps at the time of manufacturing an electrode to obtain a three-dimensional cross-linking structure body, and this binder resin prevents an active material or conductive agent from dropping off at the time of battery charge and discharge. However, with this method, cross-linking of the three-dimensional cross-linking structure body obtained by heating and pressure conditions at the time of manufacture of an electrode is insufficient, variation of cross-linking also easily occurs, and a sufficient cushioning property cannot be manifested. Furthermore, in a case where such binder of solvent-dissolution type is used, if a resin solution is coated on a substrate of an electrode, and then an organic solvent is removed, it has the problem that the surface of the electro-active material is covered with the resin without any uncovered portion and thus sufficient electrical properties are not obtained. Furthermore, there were also problems that a process of manufacturing an electrode requires accuracy and the process becomes cumbersome and complicated. Furthermore, in recent years, simplification of processes or saving energy was required due to the concern about environment and the like. For this reason, a method is desired which allows short heating treatment processes with low energy demands at the time of manufacturing an electrode. In addition, removal of an organic solvent is difficult depending on the kind of the binder resin, and the problem of odor has not been solved.
On the other hand, in Patent Literature 2, an aqueous cross-linking polymer is used in addition to an organic binder. It is inferred that kneading of the aqueous cross-linking polymer with an electrode-active material has the role of a spacer that prevents aggregation of the electrode-active material, which leads to an electrode that has an excellent cycling property. To increase this spacer effect, porous polymer particles, or hollow polymer particles are used in Patent Literature 2. However, adhesion to an electrode is insufficient, and thus a binder must be used in a large amount, which deteriorates battery performances such as an initial capacity.
Furthermore, in Patent Literature 3, polymer particles containing aqueous polymers within the particles are used, which improves the compatibility of a binder composition or reduces the leakage to an electrolytic solution, and improves binding force of a binder. However, this method also leads to insufficient adhesion to an electrode similarly to the method of Patent Literature 2, and thus does not improve the cycling life of charge and discharge.